Third generation (3G) mobile systems, such as, for instance, universal mobile telecommunication systems (UMTS) standardized within the third generation partnership project (3GPP) have been based on wideband code division multiple access (WCDMA) radio access technology. Today, 3G systems are being deployed on a broad scale all around the world. After enhancing this technology by introducing high-speed downlink packet access (HSDPA) and an enhanced uplink, also referred to as high-speed uplink packet access (HSUPA), the next major step in evolution of the UMTS standard has brought the combination of orthogonal frequency division multiplexing (OFDM) for the downlink and single carrier frequency division multiplexing access (SC-FDMA) for the uplink. This system has been named long term evolution (LTE) since it has been intended to cope with future technology evolutions.
The LTE system represents efficient packet based radio access and radio access networks that provide full IP-based functionalities with low latency and low cost. The Downlink will support data modulation schemes QPSK, 16QAM, and 64QAM and the Uplink will support BPSK, QPSK, 8PSK and 16QAM.
LTE's network access is to be extremely flexible, using a number of defined channel bandwidths between 1.25 and 20 MHz, contrasted with UMTS terrestrial radio access (UTRA) fixed 5 MHz channels. Spectral efficiency is increased by up to four-fold compared with UTRA, and improvements in architecture and signalling reduce round-trip latency. Multiple Input/Multiple Output (MIMO) antenna technology should enable 10 times as many users per cell as 3GPP's original WCDMA radio access technology. To suit as many frequency band allocation arrangements as possible, both paired (frequency division duplex FDD) and unpaired (time division duplex TDD) band operation is supported. LTE can co-exist with earlier 3GPP radio technologies, even in adjacent channels, and calls can be handed over to and from all 3GPP's previous radio access technologies.
The overall architecture is shown in FIG. 1 and a more detailed representation of the E-UTRAN architecture is given in FIG. 2. The E-UTRAN consists of an eNodeB, providing the E-UTRA user plane (PDCP/RLC/MAC/PHY) and control plane (RRC) protocol terminations towards the user equipment (UE). The eNodeB (eNB) hosts the Physical (PHY), Medium Access Control (MAC), Radio Link Control (RLC) and Packet Data Control Protocol (PDCP) layers that include the functionality of user-plane header-compression and encryption. It also offers Radio Resource Control (RRC) functionality corresponding to the control plane. It performs many functions including radio resource management, admission control, scheduling, enforcement of negotiated uplink Quality of Service (QoS), cell information broadcast, ciphering/deciphering of user and control plane data, and compression/decompression of downlink/uplink user plane packet headers. The eNodeBs are interconnected with each other by means of the X2 interface.
The eNodeBs are also connected by means of the S1 interface to the EPC (Evolved Packet Core), more specifically to the MME (Mobility Management Entity) by means of the S1-MME and to the Serving Gateway (SGW) by means of the S1-U. The S1 interface supports a many-to-many relation between MMEs/Serving Gateways and eNodeBs.
FIG. 3 illustrates structure of a component carrier in LTE Release 8. The downlink component carrier of the 3GPP LTE Release 8 is sub-divided in the time-frequency domain in so-called subframes each of which is divided into two downlink slots 320 corresponding to a time period Tslot. The first downlink slot comprises a control channel region within the first OFDM symbol(s). Each sub-frame consists of a given number of OFDM symbols in the time domain, each OFDM symbol spanning over the entire bandwidth of the component carrier. The smallest unit of resources that can be assigned by a scheduler is a resource block 130 also called physical resource block (PRB). A PRB 330 is defined as NsymbDL consecutive OFDM symbols in the time domain and NscRB consecutive sub-carriers in the frequency domain. In practice, the downlink resources are assigned in resource block pairs. A resource block pair consists of two resource blocks. It spans NscRB consecutive sub-carriers in the frequency domain and the entire 2·NsymbDL modulation symbols of the sub-frame in the time domain. NsymbDL may be either 6 or 7 resulting in either 12 or 14 OFDM symbols in total. Consequently, a physical resource block 330 consists of NsymbDL×NscRB resource elements 140 corresponding to one slot in the time domain and 180 kHz in the frequency domain (further details on the downlink resource grid can be found, for example, in 3GPP TS 36.211, “Evolved universal terrestrial radio access (E-UTRA); physical channels and modulations (Release 8)”, version 8.9.0, December 2009, Section 6.2, available at http://www.3gpp.org. which is incorporated herein by reference).
The number of physical resource blocks NRBDL in downlink depends on the downlink transmission bandwidth configured in the cell and is at present defined in LTE as being from the interval of 6 to 110 PRBs.
The frequency spectrum for IMT-advanced was decided at the World Radio Communication Conference (WRC-07) in November 2008. However, the actual available frequency bandwidth may differ for each region or country. The enhancement of LTE standardized by 3GPP is called LTE-advanced (LTE-A) and has been approved as the subject matter of Release 10. LTE-A Release 10 employs carrier aggregation according to which two or more component carriers as defined for LTE Release 8 are aggregated in order to support wider transmission bandwidth, for instance, transmission bandwidth up to 100 MHz. More details on carrier aggregation can be found in 3GPP TS 36.300 “Evolved Universal terrestrial Radio Access (E-UTRA) and Universal terrestrial Radio Access Network (E-UTRAN); Overall description”, v10.2.0, December 2010, Section 5.5 (Physical layer), Section 6.4 (Layer 2) and Section 7.5 (RRC), freely available at http://www.3qpp.org/ and incorporated herein by reference. It is commonly assumed that the single component carrier does not exceed a bandwidth of 20 MHz. A terminal may simultaneously receive and/or transmit on one or multiple component carriers depending on its capabilities. A UE may be configured to aggregate a different number of component carriers (CC) in the uplink and in the downlink. The number of downlink CCs which can be configured depends on the downlink aggregation capability of the UE. The number of uplink CCs which can be configured depends on the uplink aggregation capability of the UE. However, it is not possible to configure a UE with more uplink CCs than downlink CCs.
The term “component carrier” refers to a combination of several resource blocks. In future releases of LTE, the term “component carrier” is no longer used; instead, the terminology is changed to “cell”, which refers to a combination of downlink and optionally uplink resources. The linking between the carrier frequency of the downlink resources and the carrier frequency of the uplink resources is indicated in the system information transmitted on the downlink resources. There is a PCell (Primary Cell) and none or several (e.g. up to four) SCells (Secondary Cells). It may be noted that the different cells do not have to be tied to the same logical network element (such as an eNodeB) or physical transmission point (e.g. an antenna site); it can also be envisaged that different cells seen by a terminal are transmitted from different network elements and/or transmission points. A first example is that PCell and SCell are both tied to the same eNodeB, but are transmitted from two different transmission points, e.g. PCell from the location of the eNodeB and SCell from a remote radio-head connected to the eNodeB. Another example is that PCell is tied to and transmitted from a first eNodeB, while SCell is tied to and transmitted from a second eNodeB. It should also be noted that PCell and SCell, as well as component carriers, could be completely, partly, or non-overlapping with respect to their time and frequency transmission resources. The terms Cell and component carrier are used in the following interchangeably, since both Scells and Pcells may be seen as a component carrier. This however should not be interpreted to restrict the scope of the invention to a particular Release of the LTE standard.
When carrier aggregation is configured, the UE only has one RRC connection with the network. At RRC connection establishment/re-establishment/handover, one serving cell provides the NAS mobility information (e.g. TAI), and at RRC connection re-establishment/handover, one serving cell provides the security input. This cell is referred to as the Primary Cell (PCell). In the downlink, the carrier corresponding to the PCell is the Downlink Primary Component Carrier (DL PCC) while in the uplink it is the Uplink Primary Component Carrier (UL PCC).
Depending on UE capabilities, Secondary Cells (SCells) can be configured to form together with the PCell a set of serving cells. In the downlink, the carrier corresponding to an SCell is a Downlink Secondary Component Carrier (DL SCC) while in the uplink it is an Uplink Secondary Component Carrier (UL SCC).
The configured set of serving cells for a UE therefore always consists of one PCell and zero or more SCells:                For each SCell the usage of uplink resources by the UE in addition to the downlink ones is configurable (the number of DL SCCs configured is therefore always larger or equal to the number of UL SCCs and no SCell can be configured for usage of uplink resources only);        From a UE viewpoint, each uplink resource only belongs to one serving cell;        The number of serving cells that can be configured depends on the aggregation capability of the UE;        PCell can only be changed with handover procedure (i.e. with security key change and RACH procedure);        PCell is used for transmission of PUCCH;        Unlike SCells, PCell cannot be de-activated;        NAS information is taken from PCell.        
The reconfiguration, addition and removal of SCells can be performed by RRC. At intra-LTE handover, RRC can also add, remove, or reconfigure SCells for usage with the target PCell. When adding a new SCell, dedicated RRC signalling is used for sending all required system information of the SCell i.e. while in connected mode, UEs need not acquire broadcasted system information from the SCells.
The principle of link adaptation is fundamental to the design of a radio interface which is efficient for packet-switched data traffic. Unlike the early versions of UMTS (Universal Mobile Telecommunication System), which used fast closed-loop power control to support circuit-switched services with a roughly constant data rate, link adaptation in LTE adjusts the transmitted data rate (modulation scheme and channel coding rate) dynamically to match the prevailing radio channel capacity for each user.
For the downlink data transmissions in LTE, the eNodeB typically selects the modulation scheme and code rate (MCS) depending on a prediction of the downlink channel conditions. An important input to this selection process is the Channel State Information (CSI) feedback transmitted by the User Equipment (UE) in the uplink to the eNodeB.
Channel state information is used in a multi-user communication system, such as for example 3GPP LTE to determine the quality of channel resource(s) for one or more users. In general, in response to the CSI feedback the eNodeB can select between QPSK, 16-QAM and 64-QAM schemes and a wide range of code rates. This CSI information may be used to aid in a multi-user scheduling algorithm to assign channel resources to different users, or to adapt link parameters such as modulation scheme, coding rate or transmit power, so as to exploit the assigned channel resources to its fullest potential.
The CSI is reported for every component carrier, and, depending on the reporting mode and bandwidth, for different sets of subbands of the component carrier. A channel resource may be defined as a “resource block” as exemplary illustrated in FIG. 3 where a multi-carrier communication system, e.g. employing OFDM as for example discussed in the LTE work item of 3GPP, is assumed. More generally, it may be assumed that a resource block designates the smallest resource unit on an air interface of a mobile communication that can be assigned by a scheduler. The dimensions of a resource block may be any combination of time (e.g. time slot, sub-frame, frame, etc. for time division multiplex (TDM)), frequency (e.g. subband, carrier frequency, etc. for frequency division multiplex (FDM)), code (e.g. spreading code for code division multiplex (CDM)), antenna (e.g. Multiple Input Multiple Output (MIMO)), etc. depending on the access scheme used in the mobile communication system.
Assuming that the smallest assignable resource unit is a resource block, in the ideal case channel quality information for each and all resource blocks and each and all users should be always available. However, due to constrained capacity of the feedback channel this is most likely not feasible or even impossible. Therefore, reduction or compression techniques are required so as to reduce the channel quality feedback signalling overhead, e.g. by transmitting channel quality information only for a subset of resource blocks for a given user.
In 3GPP LTE, the smallest unit for which channel quality is reported is called a subband, which consists of multiple frequency-adjacent resource blocks.
As described before, user equipments will usually not perform and report CSI measurements on configured but deactivated downlink component carriers but only radio resource management related measurements like RSRP (Reference Signal Received Power) and RSRQ (Reference Signal Received Quality). When activating a downlink component carrier, it's important that the eNodeB acquires quickly CSI information for the newly activated component carrier(s) in order to being able to select an appropriate MCS for efficient downlink scheduling. Without CSI information the eNodeB doesn't have knowledge about the user equipment's downlink channel state and would most likely select a too aggressive or too conservative MCS for downlink data transmission, both of which would in turn lead to resource utilization inefficiency due to required retransmissions or unexploited channel capacity.
Commonly, mobile communication systems define special control signalling that is used to convey the channel quality feedback. In 3GPP LTE, there exist three basic elements which may or may not be given as feedback for the channel quality. These channel quality elements are:                MCSI: Modulation and Coding Scheme Indicator, sometimes referred to as Channel Quality Indicator (CQI) in the LTE specification and in this document        PMI: Precoding Matrix Indicator        RI: Rank Indicator        
The MCSI suggests a modulation and coding scheme that should be used for transmission, while the PMI points to a pre-coding matrix/vector that is to be employed for spatial multiplexing and multi-antenna transmission (MIMO) using a transmission matrix rank that is given by the RI. Details about the involved reporting and transmission mechanisms are given in the following specifications to which it is referred for further reading (all these documents are available at http://www.3gpp.org and incorporated herein by reference):                3GPP TS 36.211, “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical channels and modulation”, version 10.0.0, particularly Sections 6.3.3 and 6.3.4;        3GPP TS 36.212, “Evolved Universal Terrestrial Radio Access (E-UTRA); Multiplexing and channel coding”, version 10.0.0, particularly Sections 5.2.2 and 5.2.4 and 5.3.3; and        3GPP TS 36.213, “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical layer procedures”, version 10.0.1, particularly sections 7.1.7, and 7.2.        
In 3GPP LTE, not all of the above identified three channel quality elements are reported at any time. The elements being actually reported depend mainly on the configured reporting mode. It should be noted that 3GPP LTE also supports the transmission of two codewords (i.e. two codewords of user data (transport blocks) may be multiplexed to and transmitted in a single sub-frame), so that feedback may be given either for one or two codewords. Some details are provided in the next sections and in Table 1 below for an exemplary scenario using a 20 MHz system bandwidth. This information is based on 3GPP TS 36.213, Section 7.2.1 mentioned above. Codewords and their mapping on layers is described in detail for instance in 3GPP TS 36.211, Section 6.3.3.2.
The individual reporting modes for the aperiodic channel quality feedback are defined in 3GPP LTE as follows:
Reporting Mode 1-2
Contents of this Report for Transmission Modes 1-8:
                One set S MCSI value per codeword        One preferred PMI for each subband is selected        In case of transmission modes 4 or 8: One RI valueContents of this Report for Transmission Mode 9:        One set S MCSI value per codeword        One first preferred PMI for set S        One preferred PMI for each subband        One RI valueReporting Mode 2-0Contents of this Report:        One set S MCSI value        Positions of M selected subbands        One MCSI value for M selected subbands (2 bits differential set S MCSI value, non-negative)        In case of transmission mode 3: One RI valueReporting Mode 2-2Contents of this Report for Transmission Modes 1-8:        One set S MCSI value per codeword        One preferred PMI for set S        Positions of M selected subbands        One MCSI value for M selected subbands per codeword (2 bits differential to set S MCSI value, non-negative)        One preferred PMI for M selected subbands        In case of transmission modes 4 or 8: One RI valueContents of this Report for Transmission Mode 9:        One set S MCSI value per codeword        One first preferred PMI for set S        One second preferred PMI for set S        Positions of M selected subbands        One MCSI value for M selected subbands per codeword (2 bits differential to wideband MCSI value, non-negative)        One first preferred PMI for M selected subbands        One second preferred PMI for M selected subbands        In case of transmission modes other than transmission mode 4: One RI valueReporting Mode 3-0Contents of this Report:        One set S MCSI value        One MCSI value per subband (2 bits differential to set S MCSI value)        In case of transmission mode 3: One RI valueReporting Mode 3-1Contents of this Report for Transmission Modes 1-8:        One set S MCSI value per codeword        One preferred PMI for set S        One MCSI value per codeword per subband (2 bits differential to set S MCSI value)        In case of transmission modes 4 or 8: One RI valueContents of this Report for Transmission Mode 9:        One set S MCSI value per codeword        One first preferred PMI for set S        One second preferred PMI for set S        One MCSI value per codeword per subband (2 bits differential to set S MCSI value)        One RI value        
The below Table 1 discloses the amount of bits used for CSI reporting for the different Transmission Modes and Reporting Modes combinations. Whether or not the RI value is transmitted as well, is not considered in the following Table 1, i.e. the bits only cover the CSI reporting as such, MCSI (CQI) and PMI. It should be noted that for some modes detailed numbers are not yet agreed in the standard, and may thus be changed during further standardization.
As mentioned above, for this table it is assumed that the component carrier has a 20 MHz bandwidth.
TABLE 1Antenna port& rank indicatorReporting ModeTransmission Mode #conditions1-22-03-02-23-11NANA2430NANA(Single-antenna port 0)7(if the number of PBCH antennaports is one, single-antennaport, port; otherwise transmitdiversity)22TX or 4TX antennaNA2430NANA(Transmit diversity)ports32TX antenna portsNA2430NANA(Transmit Diversity if the4TX antenna ports2430associated rank indicator is 1,otherwise large delay CDD)42TX antenna ports RI = 130NANA2832(Closed-loop spatial2TX antenna ports RI > 1213261multiplexing)4TX antenna ports RI = 15632344TX antenna ports RI > 160386452TX antenna portsNANANANA32(Multi-user MIMO)4TX antenna ports3462TX antenna ports30NANA2832(Closed-loop spatial4TX antenna ports563234multiplexing with singletransmission layer)82TX antenna ports RI = 130243028322TX antenna ports RI > 12132614TX antenna ports RI = 15632344TX antenna ports RI > 160386492TX antenna ports RI = 134NANA36362TX antenna ports RI > 12540654TX antenna ports RI = 16140384TX antenna ports RI > 1644668
For instance, in transmission mode 1 and reporting mode 3-0, the CQI reporting includes 30 bits of information. In the assumed 20 Mhz component carrier scenario, for mode 3-0 there would be 13 subbands in total (100 resource blocks in total, with 8 resource blocks per subband). For each subband a differential MCSI with 2 bits is reported back. In addition, there is a wideband MCSI with 4 bits (assuming aperiodic reporting of the CSI). Therefore, the CSI feedback is composed of 30 bits.
It should be noted that the term “subband” is here used so as to represent a number of resource blocks as outlined earlier, while the term set S represents generally a subset of the whole set of resource blocks in the system bandwidth. In the context of 3GPP LTE and LTE-A, the set S so far is defined to always represent the whole cell, i.e. component carrier bandwidth, a frequency range of up to 20 MHz, and is for simplicity hereafter referred to as “wideband”. However, in the future the set S may as well only refer some of the resource blocks of the cell, in which case the skilled person shall pay attention to interpret the term wideband (or set S) used in connection with the embodiments of the invention broader than only “wideband” (or “set S) as such.
The periodicity and frequency resolution to be used by a UE to report on the CSI are both controlled by the eNodeB. The Physical Uplink Control Channel (PUCCH) is used for periodic CSI reporting only; the PUSCH is used for aperiodic reporting of the CSI, whereby the eNodeB specifically instructs the UE to send an individual CSI report embedded into a resource which is scheduled for uplink data transmission.
In addition, in case of multiple transmit antennas at the eNodeB, CSI values(s) may be reported for a second codeword. For some downlink transmission modes, additional feedback signaling consisting of Precoding Matrix Indicators (PMI) and Rank Indications (RI) is also transmitted by the UE.
In order to acquire CSI information quickly, eNodeB can schedule aperiodic CSI by setting a CSI request bit in an uplink resource grant sent on the Physical Downlink Control Channel.
In 3GPP LTE, a simple mechanism is foreseen to trigger the so-called aperiodic channel quality feedback from the user equipment. An eNodeB in the radio access network sends a L1/L2 control signal to the user equipment to request the transmission of the so-called aperiodic CSI report (see 3GPP TS 36.212, Section 5.3.3.1.1 and 3GPP TS 36.213, Section 7.2.1 for details). Another possibility to trigger the provision of aperiodic channel quality feedback by the user equipments is linked to the random access procedure (see 3GPP TS 36.213, Section 6.2).
Whenever a trigger for providing channel quality feedback is received by the user equipment, the user equipment subsequently transmits the channel quality feedback to the eNodeB. Commonly, the channel quality feedback (i.e. the CSI report) is multiplexed with uplink (user) data on the Physical Uplink Shared CHannel (PUSCH) resources that have been assigned to the user equipment by L1/L2 signalling by the scheduler (eNodeB). In case of carrier aggregation, the CSI report is multiplexed on those PUSCH resources that have been granted by the L1/L2 signal (i.e. the PDCCH) which triggered the channel quality feedback.
The content of the channel state information fields comprises different feedback elements to indicate the channel quality for a particular component carrier as already described above. According to current standardization, it may comprise one or more of the following: a modulation and coding scheme index (MCSI) value for the complete component carrier (i.e. all subbands, set S), an MCSI offset value for each subband of the component carrier (the MCSI offset value is encoded as a differential to the MCSI value of the set S of subbands), an MCSI offset value for a set M of subbands of the component carrier (set M encompasses less subbands than set S; the MCSI offset value may again be encoded as a differential to the MCSI value of the complete component carrier) and a precoding matrix indicator. A rank indicator (RI) is also transmitted for the channel state information reporting, however not within the channel state information message as such but separately, because the size of the channel status information report (MCSI & PMI) depends on the reported RI.
Which of these feedback elements is actually included into the channel state report depends amongst other things on the transmission and reporting modes configured by the base station. In each case and independent from the feedback elements included, the values of the elements in the channel state information field for the specific component carrier should be defined in a way that allows the base station to determine whether the content of said field is a genuine channel quality indication or whether it is an indication as to the status of the associated component carrier for which no channel state information was calculated.
Modulation and Coding Scheme Index (MCSI)
Adaptive modulation and coding (AMC) can be used to match the information data rate for each user to the variations in the received signal quality. The degrees of freedom for the AMC consists of the modulation and coding schemes, and the particular combination of a modulation scheme and a coding rate is indicated using the Modulation and Coding Scheme Index (MCSI). An exemplary list of modulation schemes and code rates that can be signaled by means of an MSCI is shown in Table 2 below. It should be noted that the particular entries are usually dependent on the target communication system; Table 2 shows the definition for the 3GPP LTE Release 8 system. Other systems may e.g. provide more than 16 levels or use additional or different modulation schemes.
TABLE 2MCSI (CQI)ModulationCode Rate *SpectralindexScheme1024Efficiency0Out of Range (OoR)1QPSK780.15232QPSK1200.23443QPSK1930.37704QPSK3080.60165QPSK4490.87706QPSK6021.1758716QAM3781.4766816QAM4901.9141916QAM6162.40631064QAM4662.73051164QAM5673.32231264QAM6663.90231364QAM7724.52341464QAM8735.11521564QAM9485.5547
There are several Reporting Modes and Transmission Modes that have an impact on the content of the Channel State Information reporting, and in particular on the parameters that are included to report on the channel quality of a component carrier.
In the Reporting Modes 3-0 and 3-1, one MCSI value is encoded for each subband per codeword. This is called subband differential report. MCSI value for each subbands for each codeword is encoded differentially using 2-bits relative to its respective wideband MCSI, according to the following:Subband differential MCSI offset level=subband MCSI index−wideband MCSI index
Therefore, the MCSI index for each subband can be calculated by adding the wideband MCSI index and the offset level, coded by the Differential MCSI value.
The mapping of the differential MCSI value and the actual offset level that is to be applied to the wideband MCSI index is determined by the following Table 3 for reporting modes 3-0 and 3-1.
TABLE 3Subband differentialMCSI Value (MCSIoffset subband)Offset level00112≧23≦−1
Subband size in this mode is 4, 6, or 8 RBS. For instance, if the wideband MCSI encodes MCSI index 8 (16QAM and 490/1024 code rate) and the 2-bit differential MCSI value for subband #1 is 1, then the effective MCSI index for said subband #1 is 9 (64QAM, 466/1024 code rate). When the differential MCSI value for subband #1 is 2, the resulting MCSI index for said subband #1 is at least 2 indexes higher than the wideband MCSI, i.e. ≧10.
In the reporting modes 2-0 and 2-2 one MCSI value for the M selected subbands is reported per codeword by the user equipment. subband size in this mode is 2, 3 or 4 RBS. The MCSI value for the M selected subbands for each codeword is encoded differentially using 2-bits relative to its respective wideband MCSI value according to the following:Differential MCSI offset level=MCSI index for M selected subband−wideband MCSI index
Therefore, the base station can calculate the MSCI index for the selected M subbands by adding the wideband MCSI index and the differential MSCI offset level, as encoded by the subband differential MCSI value according to the following Table 4 for reporting modes 2-0 and 2-2.
TABLE 4Differential MCSI valueMCSI Value (MCSIoffset setM)Offset level0≦112233≧4
Moreover, a spatial differential report is used in case of periodic reports for wideband and multiple codewords. The differential value ranges is −4 to +3 according toCodweword1 MCSI offset level=wideband MCSI index for for codeword 0−wideband MCSI index for for codeword 1and is shown in Table 4a.
TABLE 4aDifferential MCSI value(MCSIoffset codeword1)Offset level0011223≧34≦−45−36−27−1Precoding Matrix Indicator (PMI)
For some transmission modes, precoding feedback is used for channel dependent codebook based precoding and relies on the UEs reporting the precoding matrix indicator. Each PMI value corresponds to a codebook index according to the corresponding Tables in Chapter 6.3.4.2.3 “Codebook for precoding” of 3GPP document TS 36.211 v10.0.0. The precoder, whose index constitutes the PMI, is the precoder that maximizes the aggregate number of data bits which could be received across all layers.
As apparent from the above-mentioned tables, the PMI may be 2 or 3 bits long depending on the antenna ports used for transmission and the associated rank indicator.
Rank Indicator (RI)
The UE can also be configured to report the channel rank via a rank indicator, which is calculated to maximize the capacity over the entire bandwidth.
In particular, for spatial multiplexing, the UE shall determine a RI corresponding to the number of useful transmission layers. For transmit diversity, RI is equal to one.
In practice, the rank indicator has influence on whether the channel state information is reported for one or two codewords. For instance, the channel state information of only one codeword is reported when RI is 1, and the channel state information of two codewords is reported when RI is >1.
In the LTE system, the actual transmission rate depends on several deployment factors such as the distance between mobile terminal and base station. Thus, a denser infrastructure is required in order to support very high data rates. However, densifying the existing macro cell network tends to be rather expensive. A more attractive approach is complementing the macro cell, which provides basic coverage with additional low output power pico cells where necessary. Deployment of two or more at least partly overlaying cell layers is called heterogeneous deployment. Already in release 8 of LTE, an inter-cell interference coordination (ICIC) mechanism has been introduced in order to dynamically coordinate the resource usage among the cell layers and to avoid overlapping of the resources in different layers. The cell layers can exchange information about which frequencies they intend to schedule transmissions on in the near future which enables reduction or even complete avoidance of the additional interference. To separate control signaling for different cell layers, frequency domain schemes employ carrier aggregation. At least one component carrier in each cell layer is protected from interference from other cell layers by not transmitting control signaling on the respective component carrier. The time domain schemes employs separation of control signaling on the different cell layers in the time domain. In particular, some subframes in the low power cell layer are protected from interference. In particular, in non-protected subframes, macro cell and pico-cell eNodeB (eNB) transmit at their nominal transmit power. As a consequence, the signal from the macro eNB is seen as the dominant interference source in the pico-cell. In contrast, in protected subframes, the macro eNB is transmitting almost no power. As a consequence, the interference in the pico-cell is greatly reduced. Consequently, the SINR in the pico-cell center is much higher in comparison with the non protected subframes. This results in increase coverage area. It should be noted that the protected subframes are particularly useful in the described pico-/macro-cell situation, but can be used as well in any other multi-cell deployment in order to reduce interference to neighbouring cell(s).
In the protected subframes, the wideband and subband channel quality indication values for mobile stations in pico-cells can be assumed to be rather high. Due to frequency selectivity of the pico eNodeB (pico UE channel), several subbands are likely to be much better than the wide band average. In such a case, there may be a loss due to the absolute maximum of channel quality information level 15(cf. Table 2 above), in particular for subband channel quality indication values. This is caused by the fact that the subbands substantially better than level 15 are treated in the same manner as subbands that just reach level 15. Accordingly, the scheduler cannot distinguish between different qualities of level 15 for certain mobile stations and therefore the spectra efficiency is reduced. The scheduler is incapable of knowing how much power deboosting may be possible for still reaching level 15. The scheduler also cannot know channel capacity differences between multiple mobile stations reporting level 15, and therefore multi-user diversity cannot be optimally exploited.
FIG. 4 illustrates an example of a layered system when non protected subframes are applied and when protected subframes are applied. In particular, the macro eNB 420 as well as pico eNB 410, both transmit at their nominal transmit power, which increases the interference in the pico-cell. The macro eNB 440 transmits at very low power level, while the pico eNB 430 transmits at its nominal transmission power. As a consequence, the interference at the pico-cell is reduced.